Continuation of Grant GM 24749 is requested for experimental and theoretical studies on the selective ion permeation and gating through artificial peptide channels across lipid bilayer membranes and biological channels in excised patches of cell membranes to provide a foundation for understanding, at a molecular level, the ion selective permeation mechanisms of nerve. For artificial channels, we propose to continue studies on selectivity among monovalent cations for the Gramicidin A channel, extending these to theoretical investigations of multi-site, multi-barrier single filing models and experiments designed to distinguish between alternative models. These studies should extend the basis laid by Hille and Armstrong for using cations as "probes" of the structure of the Na ion and K ion channels of nerve, extending this to the selectivity filters as well as the bindin sites. For biological channels, we propose to characterize ion permeation and gating mechanisms of Na and acetylcholine-activated channels of nerve and muscle membranes using the excised patch technique. We will study details of the ion selectivity of the channels and construct barrier models consistent with the experimental observations. We will particularly emphasize effects on divalent cations on the permeability and open-lifetime.